Patterning: Strategies for Patterning Biomolecules with Dip‐Pen Nanolithography (Small 8/2011)

Dip‐pen nanolithography (DPN) is an atomic force microscopy (AFM)‐based lithography technique, which has the ability to fabricate patterns with a feature size down to approximately 15 nm using both top‐down and bottom‐up approaches. DPN utilizes the water meniscus formed between an AFM tip and a substrate to transfer ink molecules onto surfaces. A major application of this technique is the fabrication of micro‐ and nano‐arrays of patterned biomolecules. To achieve this goal, a variety of chemical approaches has been used. This review concisely describes the development of DPN in the past decade and presents the related chemical strategies that have been reported to fabricate biomolecular paterns with DPN at micrometer and nanometer scale, classified into direct‐ and indirect DPN methodologies, discussing tip‐functionalization strategies as well.     

Patterning of Hydrophilic and Hydrophobic Gold and Magnetite Nanoparticles by Dip Pen Nanolithography

Nanoparticles offer unique physical and chemical properties. Dip pen nanolithography of nanoparticles enables versatile patterning and nanofabrication with potential application in electronics and sensing, but is not well studied yet. Herein, the patterned deposition of various nanoparticles onto unmodified silicon substrates is presented. It is shown that aqueous solutions of hydrophilic citrate and cyclodextrin functionalized gold nanoparticles as well as poly(acrylic) acid decorated magnetite nanoparticles are feasible for writing nanostructures. Both smaller and larger nanoparticles can be patterned. Hydrophobic oleylamine or n-dodecylamine capped gold nanoparticles and oleic acid decorated magnetite nanoparticles are deposited from toluene. Tip loading is carried out by dip-coating, and writing succeeds fast within 0.1 s. Also, coating with longer tip dwell times, at different relative humidity and varying frequency are studied for deposition of nanoparticle clusters. The resulting feature size is between 300 and 1780 nm as determined by scanning electron microscopy. Atomic force microscopy confirms that the heights of the deposited structures correspond to a single or double layer of nanoparticles. Higher writing speeds lead to smaller line thicknesses, offering possibilities to more complex structures. Dip pen nanolithography can hence be used to pattern nanoparticles on silicon substrates independent of the surface chemistry.     

Development of Dip‐Pen Nanolithography (DPN) and Its Derivatives

Dip‐pen nanolithography (DPN) is a unique nanofabrication tool that can directly write a variety of molecular patterns on a surface with high resolution and excellent registration. Over the past 20 years, DPN has experienced a tremendous evolution in terms of applicable inks, a remarkable improvement in fabrication throughput, and the development of various derivative technologies. Among these developments, polymer pen lithography (PPL) is the most prominent one that provides a large‐scale, high‐throughput, low‐cost tool for nanofabrication, which significantly extends DPN and beyond. These developments not only expand the scope of the wide field of scanning probe lithography, but also enable DPN and PPL as general approaches for the fabrication or study of nanostructures and nanomaterials. In this review, a focused summary and historical perspective of the technological development of DPN and its derivatives, with a focus on PPL, in one timeline, are provided and future opportunities for technological exploration in this field are proposed.     

Biochips for Cell Biology by Combined Dip‐Pen Nanolithography and DNA‐Directed Protein Immobilization

A general methodology for patterning of multiple protein ligands with lateral dimensions below those of single cells is described. It employs dip pen nanolithography (DPN) patterning of DNA oligonucleotides which are then used as capture strands for DNA‐directed immobilization (DDI) of oligonucleotide‐tagged proteins. This study reports the development and optimization of PEG‐based liquid ink, used as carrier for the immobilization of alkylamino‐labeled DNA oligomers on chemically activated glass surfaces. The resulting DNA arrays have typical spot sizes of 4–5 μm with a pitch of 12 μm micrometer. It is demonstrated that the arrays can be further functionalized with covalent DNA‐streptavidin (DNA‐STV) conjugates bearing ligands recognized by cells. To this end, biotinylated epidermal growth factor (EGF) is coupled to the DNA‐STV conjugates, the resulting constructs are hybridized with the DNA arrays and the resulting surfaces used for the culturing of MCF‐7 (human breast adenocarcinoma) cells. Owing to the lateral diffusion of transmembrane proteins in the cell's plasma membrane, specific recruitment and concentration of EGF receptor can be induced specifically at the sites where the ligands are bound on the solid substrate. This is a clear demonstration that this method is suitable for precise functional manipulations of subcellular areas within living cells.     

Synergic Effects of Randomly Aligned SWCNT Mesh and Self‐Assembled Molecule Layer for High‐Performance,Low‐Bandgap,Polymer Solar Cells with Fast Charge Extraction

Currently, most of the promising organic solar cells (OSCs) are based on low bandgap polymer donors with deep‐lying highest occupied molecular orbit (HOMO) levels, which impose the challenges for device architecture design. In terms of fast charge extraction and suppression of bimolecular recombination, elaborate interface design in low bandgap OSCs is of significance to further boost their ultimate efficiency. In this work, a facile solution‐processed functionalized single wall carbon nanotube (f‐SWCNT) mesh/self‐assembled molecule (SAM) hybrid structure is reported as hole transport layer (HTL) in low bandgap OSCs. The effectiveness of such hybrid HTL originates from two aspects: (i) SAM layer can effectively realize Ohmic contact between f‐SWCNT and low bandgap polymer donors with deep‐lying HOMO levels due to the reduction of interface energy barrier; (ii) f‐SWCNT mesh can provide fast hole extraction pathways to quickly sweep out photogenerated charges. As a consequence of synergic effects of such hybrid HTL, both photocurrent and fill factor are greatly enhanced due to the reduced bimolecular recombination. Together with careful light management by using ZnO optical spacer, a high efficiency of 10.5% has been achieved. This work offers an excellent choice for large‐scale processable and effective HTL toward the application in low bandgap OSCs with deep‐lying energy levels.     

The Role of Liquid Ink Transport in the Direct Placement of Quantum Dot Emitters onto Sub‐Micrometer Antennas by Dip‐Pen Nanolithography

Dip‐pen nanolithography (DPN) is used to precisely position core/thick‐shell (“giant”) quantum dots (gQDs; ≥10 nm in diameter) exclusively on top of silicon nanodisk antennas (≈500 nm diameter pillars with a height of ≈200 nm), resulting in periodic arrays of hybrid nanostructures and demonstrating a facile integration strategy toward next‐generation quantum light sources. A three‐step reading‐inking‐writing approach is employed, where atomic force microscopy (AFM) images of the pre‐patterned substrate topography are used as maps to direct accurate placement of nanocrystals. The DPN “ink” comprises gQDs suspended in a non‐aqueous carrier solvent, o‐dichlorobenzene. Systematic analyses of factors influencing deposition rate for this non‐conventional DPN ink are described for flat substrates and used to establish the conditions required to achieve small (sub‐500 nm) feature sizes, namely: dwell time, ink‐substrate contact angle and ink volume. Finally, it is shown that the rate of solvent transport controls the feature size in which gQDs are found on the substrate, but also that the number and consistency of nanocrystals deposited depends on the stability of the gQD suspension. Overall, the results lay the groundwork for expanded use of nanocrystal liquid inks and DPN for fabrication of multi‐component nanostructures that are challenging to create using traditional lithographic techniques.